Broadly speaking, valves can be divided into two general types. There are those in which the orifice or port is embodied in a pipe or casing, and the port is opened or closed by insertion of, or removal of, or change in disposition of, a movable closure member. Such valves include gate, globe, butterfly, check, diaphragm, and needle valves, as well as others known to those skilled in the art.
A second broad family of valves is characterized by the provision of a movable member provided with a port so arranged that, by varying the disposition of said member usually about its central axis, e.g., by rotating a plug through substantially 90.degree., the valve may be opened or closed to the flow of fluids therethrough. Such valves are usually denominated tapered plug valves, parallel plug valves, spherical plug (or ball) valves, and disc valves, and are typified by arrangements of the type shown for example in Newton U.S. Pat. No. 2,297,161, Pratt U.S. Pat. No. 477,605, and Ohls U.S. Pat. No. Re. 24,102. This second family of valves normally includes, moreover, certain specialized valves wherein the moving member cooperates with a portion of the adjacent casing or pipe to define the port through which fluids pass in the open position of the valve, e.g., arrangements of the type shown in Reidt U.S. Pat. No. 1,451,588, and include other specialized valve configurations of the types shown for example in British Pat. No. 1,390,159 and No. 991,111 as well as conventional spool valves of the type employed in hydraulic and pneumatic applications.
The present invention is concerned with improvements in this second family of valves.
Conventional plug valves of types other than the spherical plug (or ball) valve, normally involve an elongated plug configuration which may exhibit an elongated flow passageway therethrough (as in Allen U.S. Pat. No. 2,505,270) or a passageway of substantially circular cross section extending through a tapered plug (as in the aforementioned Pratt patent) or through a parallel plug (as in the aforementioned Ohls patent). Plug valves of these general types have a number of recognized disadvantages, however, resulting in large part from the fact that comparatively complex sealing arrangements must be provided between the plug and valve housing to avoid leakage through the valve when the plug is in its closed position, and these sealing arrangements, in addition to their cost, complexity, and comparative unreliability, tend to increase the torque required to move the plug between its open and closed positions. Even though a plug of parallel or tapered configuration is easier and less expensive to manufacture than one of spherical configuration, therefore, spherical (or ball) plug valves have found increasing favor in recent years due to the fact that such spherical plug valves can be more reliably sealed by far simpler sealing arrangements consisting of unitary closed loop seals which engage the surface of the ball along a line or band of limited discrete width disposed in a comparatively simple geometry, e.g., in a single flat plane.
An outstanding advantage of spherical plug valves is that sealing arrangements can be used which are far more simple and effective than those which must be used with a parallel or taper plug valve. More particularly, the plug can be both supported and sealed by means of annular unitary seating members surrounding the respective ports in the casing and engaging the plug surface along respective sealing contours. Each seating member can be disposed in a single flat plane extending parallel with the axis of rotation, retained in the housing for example by location in a shouldered annular recess surrounding the corresponding port. Thus, the seat members, being of simple geometry and uniform radial section, are inexpensive to manufacture and easily replaceable in the valve assembly. Moreover, since the surface of the plug is engaged by the valve body only along the relatively narrow lines or bands of the sealing contours, friction is substantially reduced relative to that in a taper or parallel plug valve, and no further frictionreducing means, such as lubrication systems, are required.
A further advantageous feature of spherical plug valves is that the normals to the surface of the plug member at all points on the sealing contour are inclined at the same angle to the plane of the sealing contour. This leads to the result that, under service pressure, the loading or sealing force between the sealing member and the plug surface is substantially constant at all points around the sealing contour, and this condition of uniform loading is maintained even if the plug member is displaced downstream under the service pressure. Indeed, such displacement is intended to occur in certain types of valves, known as "floating plug" valves to improve the sealing efficiency. Accordingly, pre-loading of the seal member can be small thereby reducing the torque required to rotate the plug member, and reliable sealing can be obtained readily under all service pressure conditions.
While the art has been fully appreciative of the advantages which result from the foregoing sealing considerations in respect to spherical plug valves, e.g., lesser torque requirements, decreased cost of the seals, more reliable sealing, ease of seat replacement, etc., spherical ball valves have suffered from a fundamental disadvantage which the art has merely tolerated. More particularly, the size of the port or through-flow passageway in a ball valve is limited by the requirement that a sufficient surface area must be left in surrounding relation to the port for engagement by the seat, and if the subtending angle between the center of the ball and the lip of the port is as large as 45.degree. or greater, no surface area would be available for engagement by the seat. The subtending angle is therefore normally determined as a compromise between the desire for the largest port area possible for a given size of ball and the provision of adequate sealing surface on the exterior of the ball; and the recognized optimum subtending angle for most ball valves is conventionally 37.degree. although, in some cases, it has been increased to as much as 41.degree.. Any reduction in the subtending angle below substantially 37.degree. deoptimizes the port area for a given size of ball or plug, whereas any appreciable increase in the subtending angle above 37.degree. imposes the serious risk that the seat will be engaged and torn by the port lip as the ball is displaced between its open and closed positions.
The practical effect of the foregoing considerations is that as the required port size increases in a spherical ball valve, the diameter of the ball also necessarily increases. This in turn increases the torque requirements on the ball, since torque is proportional to the average radial distance between the axis of rotation of the ball and the location of the sealing surface in contact with the exterior of the ball, and imposes the further disadvantages that, as required port (and therefore ball) size increases, the cost of the ball itself increases due to the increased amount of material which must be employed therein, and the space required for installation of the valve also increases.
The present invention is intended to obviate the foregoing problems by the provision of plug shapes which, to a greater or lesser degree, can make use of all conventional ball valve technology including the aforementioned sealing considerations and advantages of ball valves, but which, in comparison with conventional ball valves, are capable of providing a relatively larger port area for a given horizontal or axial cross sectional diameter of the plug, require the utilization of less material in the manufacture of the plug member, provide a valve which is considerably more compact than a conventional ball valve of equivalent port area, and, for a given port area, reduce the torque required for operation of the valve.